BUG/MINOR: da: fix log-level comparison to emove annoying warning

BUG/MINOR: da: fix log-level comparison to emove annoying warning

When haproxy is run on the foreground with DeviceAtlas enabled, one
line of warning is seen for every test because the comparison is always
true even when loglevel is zero :

willy@wtap:haproxy$ ./haproxy  -db -f test-da.cfg
[WARNING] 151/150831 (25506) : deviceatlas : final memory image 7148029 bytes.
Deviceatlas module loaded.
[WARNING] 151/150832 (25506) : deviceatlas : .
[WARNING] 151/150833 (25506) : deviceatlas : .
[WARNING] 151/150833 (25506) : deviceatlas : .
^C

Don't emit a warning when loglevel is null.